To elucidate effects of environmental temperature on atrial natriuretic peptide (ANP) in experimental animals, the blood pressure (BP), heart rate (HR) and body temperature (BT) in hamsters, Guinea pigs, and rabbits under hot (33℃) and cold (l5℃) environmental conditions were measured by a biotelemetric system. Further, the ANP levels of auricular cardiocytes and plasma were examined by immunohistochemistry, electron microscopy, and radioimmunoassay in those animals under both environmental conditions. (1) Mean arterial BP, HR, BT : there was no change of HR, BP and BT in all species under cold condition, but BP of Guinea pig was significantly decreased in hot condition compared to the normal environmental condition (22℃). There was no change of HR in all species under hot condition, but BT in all animals was significantly increased under hot condition compared to the control level. (2) Plasma ANP concentration: the concentration of Guinea pig in hot condition was significantly lower than that in normal condition, but there was no change in the concentration of hamster and rabbit under hot condition. The concentration in all species under cold condition did not significantly change. (3) Immunohistochemistry and ultrastructure : ANP immunoreactivity (IR) in the auricular cardiocytes of Guinea pig after 24 hours under hot condition was weaker than that in normal condition, and ultrastructurally, the number of ANP-granules in the cardiocytes of' Guinea pig under hot condition was fewer than that in normal condition. The IR and the number of ANP-granules in the cardiocytes of hamster and rabbit under hot condition were the same level as normal condition. In cold condition, there were no changes in the IR and the number of ANP-granules of the cardiocytes in all species. The present study suggests that the circulating ANP and cardio-vascular systems in Guinea pig are greatly influenced by hot environmental condition.